When I pulled the master cylinder off to check the seal on the brake booster the flange was fluid free, so I think that should be OK.

The brake indicator lamp is now staying lit, the brakes seem to be acting fine and wasnt on after I replaced the master cylinder and bled the brakes.

I agree with vixen but check that brake fluid hasnt leaked out of the master cylinder into the booster bec ause that can make the diaphram go soft & get a hole in it

Suonds like the booster diaphram has a hole in it my sons sunny wagon had the same problem, swapped booster and fixed problem.GREG.

Strange thing, when a brake stoke is applied quickly, a burst of air sneaks under the flange that goes between the master cylinder and the booster body. I tried lubing the seal on the booster with petroleum jelly, but it still happens. Is there supposed to be a gasket in there?

Also, maybe to do with the same problem, when I apply the brake, the engine changes its sound briefly, also happens when the brake is let off, but not when the pedal is down or up all the way.

I thought all my vacuum leaks were behind me, when I choke off the carb with my hand it sucks hard and stalls, as opposed to the racing when I did have leaks.